Nanoscale corundum powders, sintered compacts produced from these powders and method for producing the same

ABSTRACT

Nanoscale corundum powders are obtained by first producing an Al 2 O 3  precursor by adding seed crystals to an aqueous solution of an aluminium compound and adding a base and then converting the Al 2 O 3  precursor into corundum by calcination at a high temperature. Before the calcination, the salts that are present in addition to the Al 2 O 3  precursor are separated off. The resulting product is calcined at temperatures of 700 to 975° C. and any fines that may be present are removed. The resulting corundum powders can be sintered at temperatures of ≦1200° C. to produce compacts or components of multiple layer systems.

This application is a 371 national stage application of PCT/EP01/08422filed Jul. 20, 2001, which claims priority of German Application No. 10035 679.6, filed Jul. 21, 2000.

The invention relates to nanosize corundum (α-alumina) powders, to aprocess for preparing them and to their processing to produce sinteredbodies.

Pulverulent corundum (α-Al₂O₃) is an important raw material forproducing aluminium oxide ceramics, which can in principle be achievedin two ways. One way starts out from shaped bodies which have been madedirectly from α-alumina powder (α-Al₂O₃ powder), while the other startsout from shaped bodies which comprise a α-Al₂O₃ precursor (for examplethe γ or θ phase) which is then converted in situ into the α-Al₂O₃phase.

In the processing of customary α-alumina powders to give dense sinteredbodies, the sintering temperature of α-alumina is, depending on theinitial particle size used, from 1300 to 1600° C. There have been, manyattempts to reduce the sintering temperature of α-alumina or to obtainthe thermodynamically stable α-Al₂O₃ phase at significantly lowertemperatures. The most important obstacle in this context is the highactivation energy of homogeneous nucleation, which is greatly delayedkinetically so that such nucleation can only be achieved from the otherAl₂O₃ phases (e.g. the γ or θ phase) at relatively high temperatures,since the diffusion coefficients are higher here. There have thereforebeen many attempts to achieve a significant reduction in thetransformation temperature by addition of nuclei; cf. EP-A-554908, U.S.Pat. No. 4,657,754 and WO 98/55400.

For example, U.S. Pat. No. 4,657,754 describes nanosize α-alumina havinga mean particle size of from 20 to 50 nm (“nanocorundum”) which isprepared by seeding, and this allows the synthesis temperature to bereduced sufficiently for α-Al₂O₃ powder having a density of 3.78 g/cm³(corresponding to 95% of the theoretical density) to be present at 1000°C.

In CN-A-1085187, Weng et al. describe another process starting out fromsalt solutions, which likewise gives nanocorundum having a diameter offrom 10 to 15 nm at synthesis temperatures of from 1100 to 1300° C.

However, a synthesis temperature of 1000° C. is too high for manypurposes, in particular for cofiring processes in microelectronics usingfoils or the sintering of pastes to a high density. The same applies tothe relatively high sintering temperature of conventional α-aluminapowder of from 1300 to 1600° C.

It has now surprisingly been found that the synthesis temperature can bereduced to values below 1000° C. by means of a particular processtechnique which gives an only weakly agglomerated nanosize α-alumina(corundum) powder which can be sintered at relatively low sinteringtemperatures. This apparently small improvement is of great industrialimportance since a significantly wider field of application can beaddressed as a result. For example, novel multilayer systems can beprocessed in a single cofiring step, for which a plurality of sinteringsteps at relatively high temperatures were necessary in the past, sinceall multilayer elements present can be densified at the lower sinteringtemperature.

The invention provides a process for preparing nanosize α-aluminapowders, in which an Al₂O₃ precursor is firstly prepared by admixing anaqueous solution of an aluminium compound with seed crystals and addinga base and then converting the Al₂O₃ precursor into α-alumina bycalcination at elevated temperature, which is characterized in that thesalts present in addition to the Al₂O₃ precursor are separated off priorto the calcination, the product obtained is calcined at temperatures offrom 700 to 975° C. and any fines (<40 nm) present are removed.

Aluminium compounds suitable for preparing the Al₂O₃ precursor arepreferably water-soluble aluminium salts such as aluminium(III) nitrate,aluminium(III) chloride, aluminium(III) acetate or aluminium(III)ethoxide.

These aluminium compounds are, for example, dissolved in deionized waterand admixed with seed crystals which preferably have a particle size of<100 nm. Examples of suitable seed crystals are α-alumina or diasporenuclei.

A base is added and, as a result, the desired Al₂O₃ precursor requitedfor conversion into α-alumina at temperatures below 1000° C. is formedduring a ripening time. Examples of bases which can be used areinorganic or organic bases such as sodium hydroxide, potassiumhydroxide, calcium hydroxide or magnesium hydroxide, ammonia, urea,aliphatic and aromatic amines, with particular preference being given tobases such as ammonia which can be separated off thermally.

The precipitation or ripening is usually carried out at temperatures offrom 50 to 100° C., preferably from 70 to 90° C. and particularlypreferably from 80 to 90° C., over a period of from 20 to 145 hours,preferably from 60 to 90 hours and particularly preferably from 70 to 80hours.

After this ripening, the preparation of nanocorundum is preferablycarried out by one of the following two alternative methods.

In method 1, the aqueous solvent is removed, preferably by freezedrying, and the salts present as impurities are decomposed thermally attemperatures of from 150 to 500° C., for example at 400° C. The productobtained is mechanically comminuted and converted into α-Al₂O₃ bycalcination at temperatures of from 700 to 975° C., preferably from 750to 950° C. and in particular from 800 to 900° C. Calcination is usuallycarried out for a period of from one to three hours.

The α-alumina powder obtained by method 1 has a high proportion ofα-alumina, but as secondary phase still contains a small proportion offines (<40 nm) which comprises mainly phases other than α-Al₂O₃. It isimportant for the purposes of the invention to remove at least most ofthese fines so that the nanosize α-alumina powder can subsequently bedensified at sintering temperatures of ≦1200° C.

The removal of the fines is preferably carried out by centrifugation.For this purpose, the α-alumina powder which has been prepared isdispersed in an aqueous solution with the aid of a dispersant (surfacemodifier) and subsequently centrifuged one or more times. Suitabledispersants are, for example, inorganic acids (preferably HNO₃),aromatic or aliphatic monocarboxylic, dicarboxylic or polycarboxylicacids, aromatic or aliphatic oxacarboxylic acids such as trioxadecanoicacid (TODA), β-dicarbonyl compounds and amino acids. The dispersantconcentration is matched to the specific surface area of the α-aluminapowder synthesized, so that, for example, 4–5 μmol of dispersant areavailable per m² of Al₂O₃ surface.

In method 2, the salt loading is reduced or removed by dialysis. Forthis purpose, the solution containing the Al₂O₃ precursor is introducedinto dialysis tubes and the latter are placed in deionized water. Thedialysed solution is subsequently frozen and freeze dried. The powderobtained can, if desired, be calcined at from 150 to 500° C. (e.g. 400°C.) to achieve complete removal of the salts still present. The powderis converted into α-Al₂O₃ as in method 1 by calcination at temperaturesof from 700 to 975° C., preferably from 750 to 950° C. and in particularfrom 800 to 900° C.

In this method 2, no or very little fines comprising non-α-Al₂O₃ phasesare formed during the synthesis, so that the α-Al₂O₃ powder obtainedcan, after surface modification using suitable surface modifiers such asinorganic acids (preferably HNO₃), aromatic or aliphatic monocarboxylic,dicarboxylic or polycarboxylic acids, aromatic or aliphatic oxacarboxylic acids, e.g. trioxadecanoic acid (TODA), β-dicarbonylcompounds or amino acids, be densified directly at sinteringtemperatures of ≦1200° C. The amount of surface modifier is matched tothe specific surface area of the α-alumina powder synthesized, so that,for example, 4–5 μmol of dispersant are available per m² of Al₂O₃surface. The surface modification can be carried out, for example, bymeans of a ball mill (3–4 h, aluminium oxide milling media ≦1 mm),mortar mills, a three-roll mill or a kneader, matched to the subsequentshaping technique.

The result is a redispersible α-alumina powder which can be processedfurther by various shaping processes such as tape casting, screenprinting, pad printing, electrophoresis, slip casting, extrusion,injection moulding. The mean primary particle size is usually from 30 to150 nm, preferably from 40 to 100 nm and particularly preferably from 50to 70 nm. The α-alumina powder is only weakly agglomerated in theredispersed state. It has a phase purity (α-Al₂O₃ content) of ≧80% byweight, preferably ≧90% by weight and in particular ≧95% by weight, anda density of ≧3.90 g/cm³, preferably ≧3.93 g/cm³, particularlypreferably ≧3.95 g/cm³.

The α-alumina powder prepared according to the invention is mixed withcustomary processing aids, e.g. organic solvents, binders, plasticizers,for further shaping. Suitable solvents are, for example, ethyleneglycol, diethylene glycol monobutyl ether and diethylene glycolmonoethyl ether, either individually or as mixtures. Examples of binderswhich could be used are cellulose derivatives such ashydroxypropylcellulose, polyvinylpyrrolidone, acrylate polymers andoligomers, methacrylates such as tetraethylene glycol dimethacrylate andpolyethylene glycol dimethacrylate. Use is made of, for example, 15% byweight of binders, based on the solid employed. Plasticizers used are,for example, polyethylene glycol dimethacrylates, polyethylene glycols(e.g. PEG 600, PEG 800, PEG 1000, PEG 2000, PEG 6000). Use is made of,for example, 25% by weight, based on the binder employed.

The nanosize α-alumina powders of the invention are suitable forproducing dense Al₂O₃ sintered bodies in the form of components orconstituents of multilayer structures. Specific applications of thesecomponents and multilayer systems are (micro)electronics, sensors (gas,pressure or piezoelectric sensors), microsystem technology (e.g.microreactors), ceramic filter elements and catalyst supports.

The following examples illustrate the invention.

EXAMPLE 1 Preparation of Nanocorundum

16 l of deionized water are placed in a stirred glass vessel, and 4 kgof Al(NO₃)₃. 6H₂O while stirring. 60 g of aluminium oxide nuclei(α-aluminium oxide or diaspore) are then added in the form of a 5–20% byweight aqueous suspension (pH>3). The mixture is heated to a temperatureof 85° C.±5° C. The pH of the solution is adjusted to 4.8±0.1 by meansof aqueous ammonia solution (25% by weight). The mixture is maintainedat a temperature of 85° C.±5° C. for 72 hours while stirring. After 72hours, two alternative ways of preparing nanosize α-alumina can beemployed.

Method 1

The “solution” obtained is frozen (for example at −30° C.) andsubsequently dried (freeze drying). The powder is then heated to 400° C.at a heating rate of 2 K/min (air atmosphere) and maintained at thistemperature for one hour. After cooling, the powder is dry milled in amortar mill for one hour. The powder is subsequently brought to 800° C.at a heating rate of 10 K/min and immediately heated to 900° C. at aheating rate of 2 K/min and maintained at this temperature for one hour.The powder prepared in this way has a specific surface area of about20–60 m²/g and a density of 3.6–3.9 g/cm³, in each case depending on thenuclei used.

After cooling, the powder is dispersed in a ball mill using aluminiumoxide milling media (≦1 mm) and an organic acid (TODA) asdispersant/surface modifier for 3–4 hours. The dispersant content ismatched to the specific surface area of the aluminium oxide powdersynthesized, so that 4–5 μmol of TODA are present per m² of Al₂O₃surface. After the tiling process, the fines in the aluminium oxidepowder obtained are separated off by multiple centrifugation. Theseparation limit in the centrifugation is, on the basis of calculation,at a particle size of about 40 nm. The fines comprise predominantly(>90%) non-α-Al₂O₃ particles. The centrifugate is freeze dried to removethe solvent.

Method 2

The “solution” obtained is purified by dialysis in portions containingabout 400 g of ammonium nitrate to remove the dissolved ammonium nitrateions. For this purpose, the solution is introduced into a dialysis tube(pore size: 2.5–3 nm) and stored in deionized water for about 2 hours,after which the water is replaced and dialysis is continued for anothertwo hours. The dialysed solution is frozen (for example at −30° C.) andsubsequently dried (freeze drying). If desired, the powder canadditionally be heated to 400° C. at a heating rate of 2 K/min (airatmosphere) and maintained at this temperature for one hour. However,this step is not absolutely necessary. The powder is subsequentlybrought to 800° C. at a heating rate of 10 K/min and immediately heatedto 900° C. at a heating rate of 2 K/min. A hold period of one hour isemployed at 900° C.

The powder prepared in this way has a specific surface area of about18–22 m²/g and a density of 3.95–3.98 g/cm³. The primary particle sizeis 40–70 nm, and the powder is weakly agglomerated in the redispersedstate.

EXAMPLE 2 Production of Sintered Aluminium Oxide Layers in MultilayerSystems

10.5 g of the α-Al₂O₃ prepared in Example 1 are homogeneously mixed with2.8 g of a 1:1 solvent mixture of ethylene glycol and diethylene glycolmonobutyl ether and 0.5 g of polyvinylpyrrolidone as binder. As mixers,it is possible to use mortars, kneaders or mortar mills. The pasteobtained is passed a number of times through a three-roll mill for finalhomogenization.

The aluminium oxide paste is applied by a thick layer method (screenprinting) to previously sintered α-alumina substrates or green(unsintered) substrates comprising yttrium-stabilized (3 mol % of Y₂O₃)zirconium dioxide in dry layer thicknesses up to 30 μm and dried so thatthey were free of cracks at 80° C. in a convection drying oven. Theprinted layers on the α-alumina substrates are densified firmly at 1200°C. (heating rate: 5 K/min) with a hold time of one hour. Densificationof the α-Al₂O₃ layers printed onto green (unsintered) substratescomprising yttrium-stabilized zirconium dioxide is carried out in twostages. In the first stage, the organics present in the composite areremoved by thermal decomposition at temperatures up to 450° C. under aprotective gas atmosphere (nitrogen). The heating time is 10 hours, holdtime: 3 hours. Thermal densification to give the dense materialcomposite is carried out in an atmosphere furnace at temperatures of1200° C., hold time: 3 hours, heating rate: 5 K/min.

EXAMPLE 3 Production of Sintered Aluminium Oxide Bodies from α-Al₂O₃Powder Prepared According to the Invention

2 g of the α-Al₂O₃ powder prepared in Example 1 are homogeneously mixedwith 1 g of a solvent mixture of ethylene glycol/diethylene glycolmonobutyl ether (1:1) and 0.15 g of a cellulose binder and dried at 100°C. 200 mg of the mixture are compacted in a uniaxial pressing toolhaving an internal diameter of 5 mm under a pressure of 200 MPa. Thecompact is subsequently after-compacted at 400 MPa in a cold isostaticpress. The pressed green body is densified thermally at 1200° C. (1 h)under an air atmosphere. After sintering, the shaped body has a densityof 3.85 g/cm³ (96.5% of theory).

1. A process for preparing a nanosize α-alumina powder, comprising:preparing an Al₂O₃ precursor by mixing an aqueous solution of analuminum compound with seed crystals and adding a base; separating anysalts present from the Al₂O₃ precursor; and calcining the Al₂O₃precursor into nanosize α-alumina powder at a temperature of from 700°C. to 975° C.
 2. The process of claim 1 wherein the aluminum compound isselected from the group consisting of aluminum(III) nitrate,aluminum(III) chloride, aluminum(III) acetate, and aluminum(III)ethoxide.
 3. The process of claim 1 wherein the seed crystals arecorundum or diaspore.
 4. The process of claim 1 wherein the base is abase that can be removed by heating.
 5. The process of claim 4 whereinthe base is ammonia.
 6. The process of claim 1 wherein the Al₂O₃precursor is precipitated at a temperature of from 50° C. to 100° C. 7.The process of claim 1 wherein any salts present are separated from theAl₂O₃ precursor by dialysis, thermal decomposition, or both dialysis andthermal decomposition.
 8. The process of claim 1 wherein the Al₂O₃precursor is calcined at a temperature of from 800° C. to 900° C.
 9. Theprocess of claim 1 further comprising: removing from the nanosizeα-alumina powder any fines present having a particle size of less than40 nm.
 10. The process of claim 9 wherein any fines present are removedby dispersion of the nanosize α-alumina powder and subsequentcentrifugation.
 11. The process of claim 1 further comprising: mixingthe nanosize α-alumina powder with at least one processing aid to give ashapeable composition.
 12. The process of claim 11 further comprisingshaping the composition; and sintering the composition.
 13. The processof claim 12 wherein the composition is sintered at a temperature of lessthan or equal to 1200° C.
 14. The process of claim 1 wherein thenanosize α-alumina powder has an α-Al₂O₃ content of at least 80% byweight and a density of at least 3.90 g/cm³.
 15. The process of claim 12wherein the shaping comprises shaping a layer of a multilayer system.16. The process of claim 15 wherein the sintering comprises sinteringthe multilayer system in a single cofiring step.